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WASHINGTON (AP) -- The Justice Department launched a broad investigation Thursday into the police department in Ferguson, Missouri, following the shooting last month of an unarmed black 18-year-old by a white police officer.

The investigation, which goes beyond an existing federal probe into the Aug. 9 shooting, will look for patterns of discrimination within the predominantly white department and focus on how officers use force, search and arrest suspects, and treat inmates at the city jail. The police department said it welcomed the investigation.

In announcing the action, Attorney General Eric Holder, who visited the St. Louis suburb two weeks ago, said he heard repeated and consistent concerns from community members about general police practices and a lack of diversity on the police force. That experience influenced the decision to seek a wide-ranging probe into the department.

"I heard from them directly about the deep mistrust that has taken hold between law enforcement officials and members of the community," Holder said, adding that other evidence reviewed so far -- including traffic stop data -- appeared to validate community concerns.

The inquiry is part of a broader Justice Department effort to investigate troubled police departments and, when pervasive problems are found, direct changes to be made. The department says it has investigated 20 police departments for a variety of systemic misconduct in the past five years, more than twice the number of cases opened in the previous five years.

Besides the investigation into the Ferguson police force, the Justice Department says it will also work with the St. Louis County police department, which trains officers from Ferguson and other local departments, to review the use of force, the handling of mass demonstrations and other aspects of policing. At the request of the county police department, federal authorities will also conduct a report on the department's response to the two weeks of sometimes violent demonstrations that followed the shooting.

The police response -- which included the use of tear gas and armored vehicles -- drew broad concern, including from members and Congress and from Holder, who said the deployment of military-style equipment sent a conflicting message.

Police have said the shooting came after a scuffle that broke out after Officer Darren Wilson told Brown and a friend to move out of the street and onto a sidewalk. Police say Wilson was pushed into his squad car and physically assaulted. Some witnesses have reported seeing Brown's arms in the air before the shooting in an act of surrender. An autopsy paid for by Brown's family concluded that he was shot six times, twice in the head.

The FBI is conducting a civil rights investigation into the shooting and a local grand jury is also evaluating the case.

The investigation announced Thursday will focus on a police department that is predominantly white, even though Ferguson is about 70 percent black. Some in Ferguson have said police disproportionately target black motorists during traffic stops, something Holder said particularly concerned him. A 2013 report by the Missouri attorney general's office found that Ferguson police stopped and arrested black drivers nearly twice as often as white motorists but were also less likely to find contraband among the black drivers.

In a statement, the Ferguson police department said it supported the investigation and was working to earn back "the trust of our residents and our neighbors."

Sen. Claire McCaskill, D-Mo., called the investigation "a step in the right direction." Civil rights advocates championed the move, too, but said more needs to be done.

"On a national level, we will remain steadfast in our commitment to eliminate police brutality everywhere and ensure the rights of those most impacted by these practices are protected in their entirety," said Tef Poe, a St. Louis artist and organizer with the group HandsUpUnited.

"This is an important step," Poe said. "However, we know much more needs to be done in order to bring the officer who killed Mike Brown to justice and address the epidemic of deadly police violence across the country."

The Justice Department's civil rights division routinely investigates individual police departments when there are allegations of systemic use-of-force violations, racial bias or other problems. The department says it is currently enforcing 14 agreements to overhaul police department practices.

In cases when an investigation discovers problematic patterns, the Justice Department will encourage significant changes to policies and practices. The investigations sometimes result in settlements known as consent decrees, in which the department agrees to make specified reforms.

"They will comb records of citizens' complaints, they will look at the filing of lawsuits, they will look at all of the record-keeping in the police department," said David Harris, a police practices expert at the University of Pittsburgh law school.

The Justice Department reached a court-supervised agreement in 2012 with the New Orleans Police Department that required the agency to overhaul its policies and procedures for use of force, training, interrogations, searches and arrests, recruitment and supervision.

In April, it issued a harshly critical report of the police department in Albuquerque, New Mexico, that faulted the agency for a pattern of excessive force and called for an overhaul of its internal affairs unit. The city and the Justice Department have been locked in negotiations over ordered changes.

The collaboration with the St. Louis County police department is being led by the Justice Department's COPS Office, which in 2012 completed a similar review of the Las Vegas Police Department.